The present invention relates to a method for the preparation of thin polyimide films by applying a solution of a polyimide prepolymer to a substrate and subsequently annealing the prepolymer.
The preparation of thin films, i.e., films with a thickness of less than or equal to about 0.2 .mu.m, often presents serious problems. Thin films are used, for example, as orientation layers in the form of so-called rubbed layers in liquid-crystal displays. For this use, the films must be transparent, possess only little intrinsic coloring and, in particular, must have a homogeneous closed surface.
To date, thin layers of organic materials are generally produced from polymers by means of the centrifuging technique, the roller-coating method or by fluidized sintering. However, sufficiently thin layers frequently are not obtained using these methods.
For preparing polyimide orientation layers for liquid-crystal displays, the procedure generally is to dissolve a polyamido carboxylic acid in a solvent and to apply it to a transparent substrate, i.e., an electrode layer or an electrode base plate. The polyamido carboxylic acid is a prepolymer for a polyimide. After the coating, the film coating is heated to an elevated temperature, i.e., annealed, whereby the polyimide is formed from the polyamido carboxylic acid (see in this connection, U.S. Pat. No. 4,068,923). However, the generally required use of highly diluted solutions causes problems with respect to the wettability and hence, the homogeneity of the layers obtained.